Why is a. Volume shader not working along with a glass BSDF shader?
There are two suzannes in the scene;
One has a  simple glass shader with a solidify modifier and is the &quot;outer suzanne&quot; as I'll call it.
The second is inside the first and has a volumetric material (the tree I used is below). Midge added a displace modifier to this one to shrink it .
Suzanne 1 (Outer) should look as so with the solidify:

Modifier stack will be as follows:

Suzanne 2 (Inner) should be as shown below, just slightly smaller than the outer suzanne to avoid any intersections that will cause shading issues will the glass material:

modifiers for inner suzanne are as follows:

This is the volumetric material I used:

Click to enlarge
The result is about as below, but any volumetric material could be used: